1. Field of the Invention
This invention relates to light emitting diodes, and to light emitting diodes with a high reflectivity contact and method for forming the contact.
2. Description of the Related Art
Light emitting diodes (LED or LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped n-type and p-type layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED.
For typical LEDs it is desirable to operate at the highest light emission efficiency, and one way that emission efficiency can be measured is by the emission intensity in relation to the input power, or lumens per watt. One way to maximize emission efficiency is by maximizing extraction of light emitted by the active region of LEDs. For conventional LEDs with a single out-coupling surface, the external quantum efficiency can be limited by total internal reflection (TIR) of light from the LED's emission region. TIR can be caused by the large difference in the refractive index between the LED's semiconductor and surrounding ambient. Some LEDs have relatively low light extraction efficiencies because the high index of refraction of the substrate compared to the index of refraction for the surrounding material, such as epoxy. This difference results in a small escape cone from which light rays from the active area can transmit from the substrate into the epoxy and ultimately escape from the LED package. Light that does not escape can be absorbed in the semiconductor material or at surfaces that reflect the light.
Different approaches have been developed to reduce TIR and improve overall light extraction, with one of the more popular being surface texturing. Surface texturing increases the light escape probability by providing a varying surface that allows photons multiple opportunities to find an escape cone. Light that does not find an escape cone continues to experience TIR, and reflects off the textured surface at different angles until it finds an escape cone. The benefits of surface texturing have been discussed in several articles. [See Windisch et al., Impact of Texture-Enhanced Transmission on High-Efficiency Surface Textured Light Emitting Diodes, Appl. Phys. Lett., Vol. 79, No. 15, October 2001, Pgs. 2316-2317; Schnitzer et al. 30% External Quantum Efficiency From Surface Textured, Thin Film Light Emitting Diodes, Appl. Phys. Lett., Vol 64, No. 16, October 1993, Pgs. 2174-2176; Windisch et al. Light Extraction Mechanisms in High-Efficiency Surface Textured Light Emitting Diodes, IEEE Journal on Selected Topics in Quantum Electronics, Vol. 8, No. 2, March/April 2002, Pgs. 248-255; Streubel et al. High Brightness AlGaNInP Light Emitting Diodes, IEEE Journal on Selected Topics in Quantum Electronics, Vol. 8, No. March/April 2002].
U.S. Pat. No. 6,657,236, also assigned to Cree Inc., discloses structures formed on the semiconductor layers for enhancing light extraction in LEDs.
Another way to increase light extraction efficiency is to provide reflective surfaces that reflect light so that it contributes to useful emission from the LED chip or LED package. In a typical LED package 10 illustrated in FIG. 1, a single LED chip 12 is mounted on a reflective cup 13 by means of a solder bond or conductive epoxy. One or more wire bonds 11 connect the ohmic contacts of the LED chip 12 to leads 15A and/or 15B, which may be attached to or integral with the reflective cup 13. The reflective cup may be filled with an encapsulant material 16 which may contain a wavelength conversion material such as a phosphor. Light emitted by the LED at a first wavelength may be absorbed by the phosphor, which may responsively emit light at a second wavelength. The entire assembly is then encapsulated in a clear protective resin 14, which may be molded in the shape of a lens to collimate the light emitted from the LED chip 12. While the reflective cup 13 may direct light in an upward direction, optical losses may occur when the light is reflected. Some light may be absorbed by the reflector cup due to the less than 100% reflectivity of practical reflector surfaces. Some metals can have less than 95% reflectivity in the wavelength range of interest.
FIG. 2 shows another LED package in which one or more LED chips 22 can be mounted onto a carrier such as a printed circuit board (PCB) carrier, substrate or submount 23. A metal reflector 24 mounted on the submount 23 surrounds the LED chip(s) 22 and reflects light emitted by the LED chips 22 away from the package 20. The reflector 24 also provides mechanical protection to the LED chips 22. One or more wirebond connections 11 are made between ohmic contacts on the LED chips 22 and electrical traces 25A, 25B on the submount 23. The mounted LED chips 22 are then covered with an encapsulant 26, which may provide environmental and mechanical protection to the chips while also acting as a lens. The metal reflector 24 is typically attached to the carrier by means of a solder or epoxy bond. The metal reflector 24 may also experience optical losses when the light is reflected because it also has less than 100% reflectivity.
The reflectors shown in FIGS. 1 and 2 are arranged to reflect light that escapes from the LED. LEDs have also been developed having internal reflective surfaces to reflect light internal to the LEDs. FIG. 3 shows a schematic of an LED chip 30 with an LED 32 mounted on a submount 34 by a metal bond layer 36. The LED further comprises a p-contact/reflector 38 between the LED 32 and the metal bond 36, with the reflector 38 typically comprising a metal such as silver (Ag). This arrangement is utilized in commercially available LEDs such as those from Cree® Inc., available under the EZBright™ family of LEDs. The reflector 38 can reflect light emitted from the LED chip toward the submount back toward the LED's primary emitting surface. The reflector also reflects TIR light back toward the LED's primary emitting surface. Like the metal reflectors above, reflector 38 reflects less than 100% of light and in some cases less than 95%. The reflectivity of a metal film on a semiconductor layer may be calculated from the materials' optical constants using thin film design software such as TFCalc™ from Software Spectra, Inc. (www.sspectra.com).
FIG. 4 shows a graph 40 showing the reflectivity of Ag on gallium nitride (GaN) at different viewing angles for light with a wavelength of 460 nm. The refractive index of GaN is 2.47, while the complex refractive index for silver is taken from the technical literature. [See Handbook of Optical Constants of Solids, edited by E. Palik.] The graph shows the p-polarization reflectivity 42, s-polarization reflectivity 44, and average reflectivity 46, with the average reflectivity 46 generally illustrating the overall reflectivity of the metal for the purpose of LEDs where light is generated with random polarization. The reflectivity at 0 degrees is lower than the reflectivity at 90 degrees, and this difference can result in up to 5% or more of the light being lost on each reflection. In a LED chip, in some instances TIR light can reflect off the mirror several times before it escapes and, as a result, small changes in the mirror absorption can lead to significant changes in the brightness of the LED. The cumulative effect of the mirror absorption on each reflection can reduce the light intensity such that less than 75% of light from the LED's active region actually escapes as LED light.